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What is Viral Encephalomyelitis (Encephalitis)?

Equine viral encephalomyelitis (encephalitis) is an inflammation of the brain and spinal cord than run concurrently or at the same time. It is an infection that stems from a virus that is transmitted by mosquitos and other blood sucking insects that can infect both humans and horses. It is an infection and inflammation which can become quite serious in both species, causing coma and death as it progresses.

Equine viral encephalomyelitis (encephalitis) is an inflammation of the brain. Found to occur mostly in various parts of North and South America, this condition is caused by three viruses (Eastern equine encephalitis virus, Venezuelan equine encephalitis virus and Western equine encephalitis virus of the genus Alphavirus).

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Symptoms of Viral Encephalomyelitis (Encephalitis) in Horses

Since the virus affects the nervous system, there some things you might notice in your horse if he is infected with equine viral encephalitis or encephalomyelitis. Here are some of those symptoms:

Fever in the range of 103 to 106 F degrees lasting 24 to 28 hours - this symptom may not be detected since it is early on and of short duration

Poor appetite

Stiffness

These are the most common early symptoms that might be noted but there are more as the disease progresses. As the disease advances, you might notice brain dysfunctional behaviors like:

Aggressiveness, irritability, excitability

Circling, head tilt, head pressing, blindness

Paralysis of pharynx, larynx and tongue

There is an incubation period of 1 to 3 weeks but sometimes the disease can progress more rapidly and the recumbency, paralysis and death symptoms and signs can be noted within 2 to 4 days following the appearance of the first signs.

Types

There are several types of equine viral encephalomyelitis and they are generally associated with areas of the country or world:

Eastern equine encephalitis (EEE) is a strain that is generally found in the Eastern United States and Canada

Western equine encephalitis (WEE) is a viral strain usually found in Argentina, Western Canada and in the United States in states located west of the Mississippi River

Venezuelan equine encephalitis (VEE) has been reported principally in Central and South America, though it has also been reported in Mexico and the United States

West Nile virus - this disease is caused by a flavivirus and presents symptoms that are similar to EEE and WEE

Causes of Viral Encephalomyelitis (Encephalitis) in Horses

The cause of these viral encephalitis diseases rests with mosquitos who carry the virus which they have harvested from the blood of wild birds. After receiving the virus, courtesy of the blood from the wild bird it feasted on, they then look for another meal of blood from an unsuspecting vertebrate like horses and humans. When they bite and ingest blood from the new vertebrate host, the virus is transmitted into the bloodstream of the host where it travels to the brain and spinal cord to inflame and infect. Since the brain is involved, the symptoms and signs are demonstrated in neurological areas as well as some flu-like symptoms being noted in humans.

Diagnosis of Viral Encephalomyelitis (Encephalitis) in Horses

While a preliminary diagnosis can be obtained by assessing the physical condition and behaviors of the horse, the travel history and the dates of that travel history, the activities of the horse and the area in which the horse was physically located when the signs and symptoms began to be noted, the final diagnosis will be based upon arboviral diagnostic testing. This testing will require laboratory evaluation of serum or cerebrospinal fluid to assess the presence of certain viral components germane to equine viral encephalitis.

If the case is a fatal one, there may be additional testing required that will only be an option for processing at certain state laboratories or the CDC labs. Your veterinarian will provide guidance for all testing to ascertain the root cause of the disease process from which your horse is suffering. Test results are generally available any time from 4 to 14 days after the specimen has been sent and received by the lab with a hard copy generated most likely around 2 weeks later. These result reporting times will fluctuate depending on the season of the year as the cases or outbreaks tend to be higher in the summer months. Your vet will want to begin treatment immediately as this is a very serious infection which can lead to death very quickly. It is important to note here that these viruses are not communicable - meaning that they cannot be passed from horse to horse or horse to human since it is a blood-borne pathogen and is transmitted by a mosquito.

Treatment of Viral Encephalomyelitis (Encephalitis) in Horses

Because equine viral encephalomyelitis (encephalitis) is caused by a virus, the treatment is mostly just supportive to keep the animal comfortable and safe. There are some medicinal interventions to help contain the pain and tremors that can accompany the disease as it progresses. Some of these steps are instituted to ensure your safety as well as that of the horse as human injury is a strong likelihood when the thrashing about occurs as the neurological deterioration aspect of disease progresses. Antibiotics may be utilized for those horses incurring injuries, pneumonia or cellulitis and basic fluids and nutrition will be needed as well.

Recovery of Viral Encephalomyelitis (Encephalitis) in Horses

Basically, the death rate statistics for horses with EEEV are 50 to 90%, for those with WEEV 20 to 50%, those with VEEV 50 to 75% and for those with flavivirus (West Nile type virus) it is 35 to 45%. Horses who do recover from EEEV, WEEV and VEEV are likely to have neurological defects which will be long lasting. Those recovering from flavivirus diseases usually fully recover without any residual deficiencies. For some horses, developing equine viral encephalomyelitis results in euthanasia which is done for humane reasons, while for others, death is spontaneous.

To prevent repeat episodes of this type of viral infection in your equine community, take all of the usual precautions to protect against mosquitoes, for example, use mosquito repellent for humans at all times, empty containers that can collect water to provide a breeding ground for mosquitoes, wear long-sleeved shirts and jackets to protect exposed skin, use fans in buildings to keep mosquitoes from landing to bite, cover your horses with fly sheets and avoid activities outside in the dawn or dusk hours. Vaccines are available for these viruses for both humans as well as equines. Talk to your veterinarian for more prevention ideas and suggestions.